Method for controlling the charging pressure at a turbocharged combustion engine, and a corresponding combustion engine

ABSTRACT

A multicylinder internal combustion engine with an exhaust-driven turbocompressor and with a divided exhaust gas flow has at least two exhaust valves ( 2,3 ) and one intake valve per cylinder. A first exhaust valve ( 2 ) is connected to a first exhaust manifold which leads to the turbine of the compressor, while a second exhaust valve ( 3 ) is connected to a second exhaust manifold which opens downstream of the turbine. The charging pressure in the engine can be regulated by virtue of the opening periods of the exhaust valves ( 2,3 ) being varied in relation to one another in order to adapt the flow through the exhaust turbine to a value which provides the desired charging pressure in the engine. By opening the second exhaust valve ( 3 ) increasingly earlier, an increasingly lower charging pressure can be achieved.

TECHNICAL FIELD

[0001] The invention relates to on the one hand a method for controllingthe charging pressure in an internal combustion engine according to thepreamble of Patent claim 1, and on the other hand an internal combustionengine according to the preamble of Patent claim 6.

STATE OF THE ART

[0002] Within vehicle technology, use is frequently made of thesupercharging of internal combustion engines in order to achieve greaterengine power at great speed. Supercharging is achieved in thisconnection by means of a compressor driven by the engine. A commonsolution is for such a compressor to be driven by an exhaust turbinedriven by the exhaust gases of the engine. One problem with thissolution is that, with a powerful exhaust gas flow, a high chargingpressure will be obtained, with an attendant risk of engine damage ifthe charging pressure becomes too high.

[0003] According to a known solution to this problem, the exhaustturbine is provided with a safety valve, what is known as a wastegatevalve, which, at a predetermined pressure in the compressor outlet inthe inlet system of the engine, reduces the pressure in the exhaustturbine by opening and discharging a certain exhaust gas flow from theexhaust turbine on to the exhaust system. In this way, the capacity ofthe compressor is reduced so that the supercharging is brought down to asafe level. One problem in this connection is that the valve usedcomplicates the turbocompressor used and is forced to work in ahigh-temperature environment. Moreover, defective functioning can havedisastrous consequences for the engine.

THE OBJECT OF THE INVENTION

[0004] One object of the invention is to eliminate the drawbacks of theknown solution and to produce a better and safer solution. Anotherobject is to achieve this using simple means.

DISCLOSURE OF THE INVENTION

[0005] These objects are achieved on the one hand by means of a methodprovided with the features indicated in Patent claim 1 and on the otherhand by means of an internal combustion engine provided with thefeatures indicated in Patent claim 6.

[0006] By making use of the divided exhaust period principle, which isknown per se, according to patent specification GB 2 185 286, where theexhaust gas flow is, by means of double exhaust valves, divided into twodifferent flows with one flow to the exhaust turbine and one flow pastthe exhaust turbine, and changing the proportions between these twoflows during operation, it is possible to change and limit the chargingpressure in the engine without the assistance of a wastegate valve inthe exhaust turbine. This is suitably achieved by changing the openingperiod of the exhaust valve which guides one flow past the exhaustturbine. By increasing this exhaust gas flow, the exhaust gas flowthrough the exhaust turbine can be reduced, with attendant lowering ofthe charging pressure in the engine.

[0007] Further advantages and features of the invention emerge from thedescription and patent claims below.

[0008] The invention is explained in greater detail below by means of anillustrative embodiment shown in the accompanying drawing.

DESCRIPTION OF FIGURES

[0009] In the drawing,

[0010]FIG. 1 shows an internal combustion engine according to theinvention, and

[0011]FIG. 2 shows the opening relationship between two exhaust valvesin a cylinder in different operating situations.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

[0012]FIG. 1 shows diagrammatically a multicylinder internal combustionengine 1 according to the invention in the form of an Otto engine. Thecylinders of the engine each have at least two exhaust valves 2 and 3.Exhaust gases are conducted out from the first exhaust valves 2 of thecylinders to a first exhaust manifold 4 common to the cylinders. Exhaustgases are conducted out from the second exhaust valves 3 of thecylinders to a second exhaust manifold 5 common to the cylinders. Thefirst exhaust manifold 4 is connected to a first catalyst 6 via a firstexhaust line 7, and the second exhaust manifold 5 is connected to thefirst catalyst 6 via a second exhaust line 8 which here contains asecond catalyst 9. One or more silencers (not shown) is or are locateddownstream of the catalyst 6 in a conventional manner.

[0013] The engine 1 is also equipped for supercharging by means of anexhaust-driven turbocompressor 10 (or turbocharger), the turbine 11 ofwhich is connected in the first exhaust line 7 and is therefore fed fromthe first exhaust manifold 4 and the first exhaust valves 2. Acompressor 12 driven by the turbine 11 supplies the engine with chargingair which, if appropriate, is cooled in a cooler 13. This charging airis fed into each cylinder in a conventional manner via one or moreintake valves 14 (not shown in detail).

[0014] The engine 1 described here therefore has a divided exhaust gasflow according to the principle described in the patent specificationmentioned, GB 2 185 286. The interaction between the two exhaust valves0.2 and 3 in each cylinder is shown in diagrammatic form in FIG. 2. Thehorizontal axis represents the crankshaft angle CA of the engine, whilethe vertical axis represents the value of the valve lift VL. The normalmovement of the two exhaust valves is shown by curves 2, 3 (solidlines). As can be seen, the two exhaust valves 2 and 3 are open with acertain stagger in relation to one another during the exhaust stroke ofthe piston between a bottom dead center position A and a top dead centerposition B. The first exhaust valve 2 opens before the piston hasreached its bottom dead center position A and closes before the pistonhas reached its top dead center position B. On the other hand, accordingto the normal case shown by the solid line, the second exhaust valve 3does not open until the piston has passed its bottom dead centerposition A and remains open for a period after the piston has passed itstop dead center position B.

[0015] According to the invention, the opening period of the secondexhaust valve 3 can be changed in such a manner that, when necessary, itopens earlier and more than in the normal case shown by the solid line.Examples of such a changed opening period are shown by the curves 3′, 3″and 3′″ (broken lines), the curves in turn showing an increasingly longopening period. By increasing the opening period of the second exhaustvalve 3 in this way, an increasing quantity of exhaust gases from theengine can be led past the turbine 11, with the result that the power ofthe latter is reduced and a smaller quantity of air is fed into theengine by means of the compressor 12. This in turn leads to thepossibility of regulating the charging pressure in the engine to adesired level by suitable selection of opening period of the secondexhaust valve 3. In this way, it is possible to regulate the chargingpressure without using a previously customary wastegate valve in theturbine.

[0016] Changing the movement of the second exhaust valve 3 can takeplace in a number of different ways, as required and desired. Onepossibility is, for example, to use variable geometry on a camshaftintended for the exhaust valves. Another possibility is to useelectromechanically operating valve depressors which can be electricallycontrolled as required. If appropriate, these solutions can be combinedor replaced by other solutions available to the person skilled in theart, all within the scope of the invention.

[0017] In addition to the solution shown here of advancing the openingof the second exhaust valve 3, it is also possible to vary the closingof the first exhaust valve 2. If appropriate, these solutions can becombined. In this connection, a technique similar to that used for thesecond exhaust valve 3 can be used for acting on the first exhaust valve2.

[0018] According to the invention, it is therefore a matter ofregulating the opening periods of the two exhaust valves 2, 3 inrelation to one another so that the exhaust gas flow through the exhaustturbine is adapted to a value which provides the desired chargingpressure in the engine.

[0019] For various reasons, it is becoming increasingly common at highpower output to drive engines with an increasingly small fuel excess,with the result that a high exhaust gas temperature is obtained. Thesolution proposed according to the invention eliminates the difficultyof bringing about good cooling of a wastegate valve in the turbine,which is exposed to increasingly high exhaust gas temperatures. Goodcooling of the first exhaust valve 2 can instead be ensured by suitabledesign of water cooling ducts around this valve in the engine.

[0020] The second catalyst 9 is suitably positioned close to the engineand is intended for a limited exhaust gas flow. When the engine isstarted, the first exhaust valve 2 is suitably kept closed for a shortperiod so as to allow all exhaust gases to pass through the secondcatalyst 9 and heat the latter up rapidly and in this way achieveeffective exhaust gas purification. In order for it to be possible,according to the invention, to receive an increased exhaust gas quantityin the case of high engine loading, the second catalyst 9 has to have asize adapted for such an increased exhaust gas flow.

[0021] The invention can of course also be applied in an embodimentwhere there is no second catalyst 9.

1. A method for controlling the charging pressure in an exhaust-driventurbocompressor (10) connected to an internal combustion engine, wherethe engine has a number of cylinders and a divided exhaust gas flow andalso has at least two exhaust valves (2, 3) and one intake valve (14)per cylinder, namely a first exhaust valve (2) connected via a firstexhaust manifold (4) to the exhaust-driven turbine (11) of theturbocompressor, and a second exhaust valve (3) connected via a secondexhaust manifold (5) to the exhaust system of the engine downstream ofthe exhaust turbine, characterized in that the opening periods of theexhaust valves (2, 3) are varied in relation to one another in order toadapt the exhaust gas flow through the exhaust turbine (11) to a valuewhich provides the desired charging pressure in the engine.
 2. Themethod as claimed in claim 1, characterized in that the second exhaustvalve (3) is opened increasingly earlier the lower the charging pressurethat is desired.
 3. The method as claimed in claim 1 or 2, characterizedin that the valve lift of the second exhaust valve (3) is made greaterthe earlier it opens.
 4. The method as claimed in any one of claims 1-3,characterized in that the second exhaust valve (3) is closed at the sametime irrespective of the desired charging pressure.
 5. The method asclaimed in any one of claims 1-4, characterized in that the firstexhaust valve (2) opens and closes in the same manner irrespective ofthe desired charging pressure.
 6. An internal combustion engine with anumber of cylinders and with a divided exhaust gas flow, provided withat least two exhaust valves (2, 3) and one intake valve (14) percylinder, namely a first exhaust valve (2) connected via a first exhaustmanifold (4) to the inlet of an exhaust-driven turbine (11) in aturbocompressor (10) for supercharging the engine, and a second exhaustvalve (3) connected via a second exhaust manifold (5) to the exhaustsystem of the engine downstream of the exhaust turbine (11),characterized in that the exhaust valves (2, 3) are made with openingperiods which are variable in relation to one another in order for it tobe possible to adapt the exhaust gas flow through the exhaust turbine toa value which provides the desired charging pressure in the engine. 7.The internal combustion engine as claimed in claim 6, characterized inthat only the second exhaust valve (3) has a variable opening period andis arranged so as to have a shorter opening period for a higher chargingpressure than for a lower charging pressure.
 8. The internal combustionengine as claimed in claim 6 or 7, characterized in that the secondexhaust valve (3) is arranged so as to open increasingly earlier for adecreasing charging pressure.
 9. The internal combustion engine asclaimed in any one of claims 6-8, characterized in that the secondexhaust valve (3) is arranged so as to have an increasing liftingmovement for a decreasing charging pressure.
 10. The internal combustionengine as claimed in any one of claims 6-9, characterized in that thesecond exhaust valve (3) has a closing point which is unchanged fordifferent charging pressures.